Thiosulphate with Iodic Acid. 241 



formed in the usual manner except that the volume was 

 adjusted just before the addition of the iodic acid, and the 

 iodine that was set free after the formation of the first reading 

 tint was destroyed at fixed intervals with measured amounts of 

 sodium thiosulphate. The results are given in Table Y. 



Table V. 



Effect of Dilution and Lapse of Time on the " After Coloration" 



Na 2 S 2 3 HI0 3 



.taken, introduced. Na 2 S 2 3 introduced. Volume. 



cm 3 . cm 3 . cm 3 . cm 3 . 



lh. 2h. 



15min. 45 min. 45 min. 45min. 20 h. Total 



( 1) 6 27*68 0-25 0*13 (W>8 00 0-03 0"49 50 



( 2) 6 27-70 20 0*10 0'03 0'03 0-03 0'39 50 



( 3) 6 28-17 0-16 0-10 0*03 01 none. 0*30 50 



( 4) 6 27-03 0-60 0-26 009 03 none. 0'98 150 



( 5) 6 27-60 0-93 0'28 06 0*04 0'04 135 150 



( 6) 6 28-60 1-34 046 0-17 0-03 0-14 2*14 200 



( 7) 6 28-85 1-20 50 0-28 06 0-27 2*31 200 



( 8) 6 31-63 1-46 0'74 0-10 021 0-23 2-74 250 



( 9) 6 2990 1-04 60 0'23 0'15 0'46 2-48 250 



(10) 6 3609 1-60 1-23 0'63 0-34 0'18 3-98 300 



(11) 6 37-59 1-65 1*33 0'72 0*27 0-10 407 300 



(12) 6 37-23 1-92 1'05 0-64 0"33 * 300 



In the experiments with small volumes the evolution of 

 iodine in any considerable quantity ceased after two or three 

 hours, although the solution would become recolored as often 

 as it was bleached for a number of days. The traces of iodine 

 thus set free, however, were seldom equivalent to more than 

 one or two drops of sodium thiosulphate. The larger volumes, 

 however, continued to separate iodine in abundance for a very 

 long time. The amount of iodine thus liberated after the first 

 coloration evidently varies with the amount of iodic acid 

 required for the titration, although not strictly proportional to 

 it. Both of these quantities increase at a regular rate with the 

 volume of the solution. 



To show with what accuracy the reaction between sodium 

 thiosulphate and iodic acid may be applied to the direct esti- 

 mation of one of these substances by the other, the averaged 

 results of a large number of titrations are compared in Table 

 YI. The operations were conducted as directed by Riegler, 

 equal measured volumes of standardized sodium thiosulphate 

 being titrated with iodic acid of known strength, in the 

 presence of starch and under different conditions of time, 

 dilution and mass, the volume of iodic acid required to pro- 

 duce the blue coloration being in each case compared with the 

 volume theoretically required by the terms of Riegler's equation. 



* No observation. 



